Carbene dimerization

Carbene dimerization is a type of organic reaction in which two carbene or carbenoid precursors react in a formal dimerization to an alkene. This reaction is often considered an unwanted side-reaction but it is also investigated as a synthetic tool. In this reaction type either the two carbenic intermediates react or a carbenic intermediate reacts with a carbene precursor.^[1] An early pioneer was Christoph Grundmann reporting on a carbene dimerisation in 1938.^[2] In the domain of persistent carbenes the Wanzlick equilibrium describes an equilibrium between a carbene and its alkene.

A reoccurring substrate is a diazo compound and more specifically an alpha-carbonyl diazo compound. For example, ethyl diazoacetate is converted to diethyl maleate using the ruthenium catalyst chloro(cyclopentadienyl)bis(triphenylphosphine)ruthenium:^[3]

Grubbs' catalyst is also effective ^[4] In this reaction type the active intermediate is a transition metal carbene complex. A diazo cross-coupling reaction has also been reported between ethyl diazoacetate and methyl phenyldiazoacetate using the rhodium catalyst [Rh₂(OPiv)₄].^[5]

A direct metal carbene dimerization has been used in the synthesis of novel Polyalkynylethenes ^[6]

References

1. March, Jerry (1985). Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (3rd ed.). New York: Wiley. ISBN 9780471854722. OCLC 642506595.
2. Grundmann, C. (1938), Über die Zersetzung der Diazoketone. Justus Liebigs Annalen der Chemie, 536: 29–36.doi:10.1002/jlac.19385360103
3. Highly stereoselective formation of cis-enediones from α-diazo carbonyl compounds catalysed by [RuCl(η5-C5H5)(PPh3)2] Walter Baratta and Alessandro Del Zotto Chem. Commun., 1997, 2163-2164 doi:10.1039/A706459D
4. Maleates from diazoacetates and dilactones from head-to-head dimerisation of alkenyl diazoacetates using Grubbs' 2nd-generation ruthenium carbene catalyst David M. Hodgson and Deepshikha Angrish Chem. Commun., 2005, 4902-4904 doi:10.1039/B510829B
5. Hansen, J. H., Parr, B. T., Pelphrey, P., Jin, Q., Autschbach, J. and Davies, H. M. L. , Rhodium(II)-Catalyzed Cross-Coupling of Diazo Compounds. Angewandte Chemie International Edition, n/a. doi:10.1002/anie.201004923
6. Barluenga, J., de Sáa, D., Gómez, A., Ballesteros, A., Santamaría, J., de Prado, A., Tomás, M. and Suárez-Sobrino, A. (2008), Metal Carbene Dimerization: Versatile Approach to Polyalkynylethenes. Angewandte Chemie International Edition, 47: 6225–6228. doi:10.1002/anie.200801584